1. Field of the Invention
The present invention relates to an electron-emitting device, and in addition, to an image-forming apparatus which uses the electron-emitting device.
2. Related Background Art
Two types of electron-emitting devices are known conventionally: the thermoelectron source type and the cold cathode electron source type. Of these, the cold cathode electron source type refers to devices including field emission type (hereinafter referred to as the FE type) electron-emitting devices, metal/insulation layer/metal type (hereinafter referred to as the MIM type) electron-emitting devices, surface conduction type electron-emitting devices, and the like.
In order to apply these types of electron-emitting devices in to an image-forming apparatus, an emission current that causes a phoshor to emit light with a sufficient luminance is necessary. Further, to obtain high definition display, electron beams to be irradiated to the fluorescent material are required to have a small diameter. In addition, ease of manufacture is also very important factor.
A Spindt type electron-emitting device is an example of a conventional FE type device. A structure in which a microtip is formed as an emission point, and electrons are emitted from a leading edge of the microtip is general for the Spindt type. Thermal destruction of an electron-emitting portion is induced, and the lifetime of an FE type device becomes limited, if the emission current density for causing the fluorescent material emit light is made large. Furthermore, there is a tendency for electrons emitted from the leading edge spread out due to an electric field formed by a gate electrode, and the beam size cannot be made smaller.
Various examples are proposed as individual solutions in order to overcome those kinds of FE type device disadvantages.
An example of disposing a convergence electrode above the electron-emitting portion is shown as an example of preventing electron beam expansion. This is generally performed by narrowing down an emitted electron beam by using a negative electric potential of the convergence electrode, but the manufacturing process thereof becomes complex and this invites a large increase in manufacturing costs.
An example disclosed in JP 8-115654 A is shown in FIG. 13. This is a structure in which an electron-emitting surface exists in a location within a microscopic opening deep from a surface of an electron-emitting substrate insulating layer. In FIG. 13, a cathode electrode 32, an insulating layer 33, and a gate electrode 34 are formed on a substrate 31, and an electron-emitting material 35 is disposed within a microscopic opening 36. The electron-emitting material 35 is disposed in a trenched portion in the cathode electrode 32, so that it is disposed at apposition which is deep from a boundary between the cathode electrode 32 and the insulating layer 33. Reference numeral 37 denotes an anode electrode with which emitted electrons collide.
In this structure, electron emission is performed from a thin film disposed within the trenched opening, and therefore there is an advantage in that a flat and equipotential surface is formed on the electron-emitting surface and spreading of the electron beam becomes smaller. Further, by using a low work function structuring material as an electron-emitting substance, electron emission is possible even if a microtip is not formed. As a result, low voltage drive is achieved. Furthermore, there is an advantage in that the manufacturing method is relatively simple. In addition, electron emission is performed by a surface, and therefore electric field concentration does not occur, tip destruction does not occur, and it has a long life.
In the above-mentioned structure, a step distance between the cathode electrode 32 and the electron-emitting material 35 largely depends upon an electric field applied to a surface of the electron-emitting material 35, and therefore it is necessary to accurately control the step difference.
Further, FIG. 14A and FIG. 14B show an example disclosed in JP 10-125215 A. This structure is also a technique of forming the electron-emitting material 35 inside the cathode electrode 32. In this example, for cases of forming the electron-emitting material 35 in a step portion, it is necessary to consider the trajectory of the electron beam in the step portion. Further, although it is appropriate that the electron-emitting material 35 only exist on a bottom face of the trenched portion in the cathode electrode 32 in the opening, cases in which the electron-emitting material 35 remains in a side wall portion of the opening by manufacturing method can also be considered.
There are cases in which the electron-emitting material 35 remaining in the side wall portion obstructs an electron emission, or causes reduction of the insulating property between the cathode electrode 32 and the gate electrode 34. In particular, a leak current flows between the cathode electrode 32 and the gate electrode 34 if the electron-emitting material 35 is a conductor, and this becomes a primary factor in reducing the electron emission efficiency.
An objet of the present invention is to provide an electron-emitting device in which electron beam size can be made smaller, and an image-forming apparatus that uses the electron-emitting device.
In order to achieve the aforementioned objective, an electron-emitting device according to the present invention comprises:
a cathode electrode;
an insulating layer;
a gate electrode;
a substrate on which laminated the cathode electrode, the insulating layer and the gate electrode;
an opening penetrating the insulating layer and the gate electrode; and
an electron-emitting material disposed within the opening;
wherein an opening bottom face is formed of a portion of the cathode electrode exposed through the opening and is elevated on a central portion compared with a peripheral portion; and
a surface of the electron-emitting material existing at least the elevated central portion of the opening bottom face is positioned lower than the height of a boundary between the cathode electrode and the insulating layer.
It is preferable that the surface area of the electron-emitting material is substantially equal to, or less than the area of the opening region penetrating the gate electrode.
It is preferable that the electron-emitting material be a substantially flat film.
With this an electron-emitting device having a small beam size can thus be structured. In addition, a structure of an electron-emitting device having low leakage is also possible.
It is preferable that the electron-emitting material be at least one material selected from the group consisting of diamond, diamond like carbon, a carbon nano-tube, and a graphite nano-fiber, having a low work function.
With this, an electron-emitting device capable of low voltage drive can thus be structured.
It is preferable that a portion of the cathode electrode existing the central portion be separated by the peripheral portion of the opening bottom face, and be electrically connected by the electron-emitting material laminated on the peripheral portion of the opening bottom face.
In this case, the structure becomes one in which a limiting resistance is added to the electron-emitting material, and the electron emission current can be stabilized.
It is preferable that the electron-emitting material be a higher resistance film than the cathode electrode.
It is preferable that the cathode electrode be structured from a plurality of layers having different resistivities.
It is preferable that the electron-emitting material be formed while being sandwiched by the cathode electrodes.
In this case, the electron-emitting material is laminated in advance, and therefore influence of the leak current can also be avoided.
An image-forming apparatus of the present invention is characterized by comprising an electron source using the electron-emitting device of the present invention, and an image-forming material which forms an image by using electrons emitted from the electron source.
The electron source and the image-forming apparatus, to which the electric field emission device according to the present invention is applied, can therefore be formed in higher definition.